The therapeutic benefits attributable to hyperthermia, i.e. elevated body temperature, as a treatment for many types of bodily disorders have been known for centuries. The common fever elegantly exemplifies the therapeutic benefits of elevated body temperature. In the presence of infection by common infectious agents, e.g., cold viruses, the human immune system responds by elevating the body's core temperature to aid in the resolution of the infection.
Throughout history, practitioners have sought to induce fever or an artificial state of fever in order to increase body temperature to treat bodily disorders. An early form of treatment for syphilis illustrates this fact. Historically, practitioners attempted to artificially elevate the body temperature of an individual infected with syphilis by such medieval methods as infecting the patient with an infectious agent, such as malaria, thereby to induce a fever in an attempt to cure the syphilis. This method, however, even if successful in curing the syphilis, left the patient chronically infected with the incurable disease of malaria.
Modern approaches toward achieving localized hyperthermia in biological tissues include dielectric heating, microwave diathermy, ultrasonic heating, and the application of hot compresses or "hot packs". All of these methods have particular clinical applications, but each exhibits one or more disadvantages.
Dielectric heating involves placing capacitative electrodes around the tissue to be treated. An alternating R.F. (radio frequency) current of between 10.sup.6 Hz to 10.sup.9 Hz is applied across the tissues and induces a dielectric loss within the tissues. This dielectric loss within the tissues results in heat generated within the tissues themselves. The excellent tissue penetration afforded by this heating modality make it an obvious choice for the deep heating of tissues. However, the need for accuracy in the placement of the electrodes and the attendant loss of control of the field location within the tissues associated with electrode placement, make heating a specific region of tissue difficult.
Microwave diathermy is an extension of the dielectric heating method. Microwave diathermy employing R.F. frequencies in the range of 10.sup.9 Hz to 10.sup.10 Hz were found to be very efficacious, but the result were plagued by various troubling effects which often outweighed the benefits. Human tissues exhibit a highly variable impedance or radiation resistance to the particular wavelengths of electromagnetic radiation employed in microwave diathermy. This variability in impedance may cause constructive interference, destructive interference, or even resonance within the tissues resulting in damaging localized heating or pain. Additionally, if the subject undergoing microwave diathermy treatment has any metallic implants such as hip joint replacements plates or pins, these implants would further concentrate the radiation and cause damage to local tissues. Microwave diathermy was commonly used until concerns over the effects of high electromagnetic (e.m.) field intensities on biologic subjects dampened the enthusiasm for this modality.
Ultrasonic diathermy is a commonly employed heating modality used in physical therapy settings to locally heat sub-surface or deep tissues to provide pain relief. These ultrasonic devices emit energy at frequencies from between 2.times.10.sup.4 to 10.sup.7. Heating of the tissues occurs because of visco-elastic loss within the tissues in response to mechano-elastic waves coupled into the tissues from an ultrasonic transducer. The variability of the impedance of human tissues can give rise to reflections at tissue interfaces such as at the muscle-bone interface or muscle-fat interface. These reflections can result in localized concentrations of energy referred to as "hot spotting" which can lead to tissue damage or subject the patient to unnecessary pain. In addition, in order to implement this modality of heating tissue, the transducer must be continuously moved over the treatment area and a messy coupling gel must be applied to the patient in order for the patient to receive the ultrasonic waves.
Both the radio frequency diathermy devices and ultrasonic diathermy devices are relatively complex and costly to manufacture and do not lend themselves to unsupervised or non-medical applications of treatment.
People use heating pads and "hot packs" for treating a variety of aches, pains, and ailments. The benefits of these devices are limited; however, since the heat is deposited on the surface of the skin and must penetrate to the underlying tissues by conduction, effective treatment is limited skin tolerance to elevated temperatures.
Infrared radiation provides another modality for inducing localized hyperthermia in biological tissues. The epidermal tissue of the skin absorbs much of the infrared radiation of wavelengths shorter than 3 .mu.m. In order to increase the internal temperature of biological tissues and, thereby, to induce localized hyperthermia without causing substantial heating of the epidermal tissue, application of infrared radiation of wavelengths in the range of 3-30 .mu.m to the biological tissues are desired. Infrared radiation in the range of 3-30 .mu.m is only partly absorbed by water molecules. Since epidermal tissue contains a greater proportion of water molecules than does deeper tissues such as fat, the infrared radiation in this range is not substantially absorbed by the epidermal tissue of the skin. A typical example of an infrared radiation source used for localized heating of biological tissues is a common heat lamp. The common heat lamp provides an inexpensive mechanism for generating infrared radiation. However, as is the case with all other prior art infrared radiation sources used for inducing localized hyperthermia, the prior art infrared sources generate a broad spectrum of infrared radiation including short wavelength infrared radiation of wavelengths less than 3 .mu.m which overheat the epidermal tissues, i.e., the skin.
U.S. Pat. No. 4,489,234 to Harnden, Jr. et al. teaches a particular infrared wavelength heating and/or cooking device utilizing honeycomb tube members for supporting glass cover plates. However, the device produces infrared radiation of wavelengths substantially less than 3 .mu.m. The wavelengths generated are best suited to heating and/or cooking but would cause serious damage if applied to biological tissues. Unfortunately, both the epidermal tissues, i.e. the skin, and the underlying tissues absorb infrared radiation of wavelengths shorter than 3 .mu.m. As a result, the prior art infrared radiation generating devices detrimentally heat the epidermal tissues of the skin and cause the temperature of the skin to increase to levels causing discomfort or epidermal tissue damage. Because the epidermal tissue of the skin absorbs infrared radiation of wavelengths less than 3 .mu.m as well as do the internal tissues, the prior art infrared hyperthermia devices limit administration of infrared radiation to less than the efficacious amounts necessary for achieving therapeutic benefit.
Medium and long wavelength infrared emitters are commonly used in industry to cure coatings and heat products. These infrared emitters typically utilize a heated, highly emissive surface to couple radiation through the air to object being heated. However, convective coupling of the air surrounding the emitter heats the air which can heat nearby objects by convection and conduction.
Lasers are an ideal source of monochromatic long wavelength infrared radiation. In particular, CO.sub.2 lasers are a good source of monochromatic long wave infrared radiation. However, lasers of this type produce a very intense and discrete beam of focused infrared radiation which could cause tissue damage. These intense and discrete beams of infrared radiation could be diffused by expanding or scanning the beam, but this reduces the power intensity (W/m.sup.2) making the laser a less effective modality of treatment. Lasers emit essentially at one wavelength. The disadvantage of emitting only a single wavelength, as opposed to a broad spectrum emitting source, is that the wavelength may be absorbed by a particular molecule or tissue. A broad spectrum emitting source increases the probability that the desired wavelengths will reach the tissues to be treated. Additionally, at this time, the high cost of the equipment, the complexity of the apparatus, i.e., sealed sources and infrared optics, and the need for trained operators preclude lasers from being a practical modality for generating long wavelength infrared radiation for inducing localized hyperthermia.
Efficiency of the radiation source is another significant problem associated with prior art radiant infrared wavelength generating devices. Typically, radiant infrared generating sources must be maintained in a temperature range of 300.degree.-3000.degree. K. to maximize radiative emission of infrared radiation. In particular, a radiation source designed to operate in the temperature range of 300.degree.-3000.degree. K. will lose efficiency due to convective heat loss to the surrounding air. This loss of efficiency due to convective and conductive heat loss requires a large input of electrical power to constantly maintain the temperature of the radiation generating source. Prior art devices have attempted to remedy this problem by operating the radiation source in a vacuum, i.e., an air-free environment. Typically, the radiation source is operated within a sealed and air-free or vacuum environment enclosed behind an infrared transparent window. While vacuum sealing is an effective mechanism for increasing the efficiency of a radiant infrared wavelength generating source for producing infrared radiation of &lt;3 .mu.m, however, longer wavelength infrared radiation in the range of 3-30 .mu.m is absorbed by most practical window materials. Therefore, using this means for generating infrared radiation having wavelength substantially in the range of 3-30 .mu.m for heating internal biological tissues with minimal heating of the external tissues is not practicable. While using a vacuum is effective at reducing convective and conductive heat losses from the radiation generating source, its use is impractical because most practical materials will not allow for the transmission of infrared wavelengths in the range of 3-30 .mu.m. Window materials suitable for transmission of medium and long wavelength infrared radiation are available such as silicon, germanium, zinc selenide and other exotic materials. These materials, however, are physically unable to withstand the pressures associated with maintaining a vacuum in a sealed, evacuated radiation source.
In order to maximize the efficiency of a radiant radiation generating source capable of emitting infrared radiation of wavelengths in the range of 3-30 .mu.m, it is necessary to find an alternative means for increasing efficiency of the radiation source capable of both preventing convective heat losses from the radiation generating source by inhibiting air flow at the radiation source and, at the same time, allows infrared radiation of wavelengths in the range of 3-30 .mu.m to pass through it.